Aluminum nitride has a high thermal conductivity and an excellent electrically insulating property, and is used for such as a highly thermally conductive substrate, a heat-radiating product, and a filler for insulating heat radiation. In recent years, semiconductor electronic components, including IC and CPU mounted on high performance electronic devices represented by such as laptops and information terminals, have been downsized and highly integrated. Heat-radiating members have been accordingly required to be smaller. Heat-radiating members used for them include, for example, a heat-radiating sheet or a film spacer where a high heat conductive filler is filled in matrix such as resin and rubber (Patent Document 1), a heat-radiating grease with fluidity made by filling a high heat conductive filler in silicone oil (Patent Document 2), a heat-radiating adhesive where a high heat conductive filler is filled in epoxy resin (Patent Document 3). As a high heat conductive filler, aluminum nitride, boron nitride, alumina, magnesium oxide, silica, graphite, various metal powder, and the like are used.
By the way, to improve heat conductivity of heat-radiating members, it is important that a filler having a high thermal conductivity is densely filled, and thus aluminum nitride powder composed of spherical aluminum nitride particles of several μm to several tens μm is desired. However, aluminum nitride powder produced by a typical method has many particles of submicron order, and thus it is difficult to obtain aluminum nitride particles having a large particle size of several tens μm or so.
A method for making spherical aluminum nitride powder is disclosed as follows.
For example, Patent Document 4 discloses a method for producing spherical aluminum nitride particles by reductive nitrogenation of spherical alumina particles. Also, Patent Document 5 discloses a method for sintering spherical granulated powder obtained by adding a sintering assistant, a binder agent, and a solvent to aluminum nitride powder and performing spray dry thereof. Further, Non Patent Document 1 discloses that aluminum nitride powder is made spherical by being heat treated in a flux composed of a precursor, such as oxide or nitride of alkaline earth element or rare earth element and salt, hydroxide, halogenide, or alkoxide that generates the oxide or nitride due to decomposition during heating, or by directly conducting heat treatment of an aluminum-nitride based composition compounded due to adding a flux component thereto, and the flux is dissolved to perform isolation.
However, in the method of Patent Document 4, cavities are generated in particles with rise in conversion into aluminum nitride, and thus aluminum nitride powder is difficult to be a true sphere. Even if aluminum nitride powder can be a nearly true spherical shape, the aluminum nitride powder has a problem that strength against the crushing of the particles is small due to the cavities and bubbling is easy to occur when the aluminum nitride particles are filled in resin. On the other hand, in the method of Patent Document 5, a spherical granulated product where a sintering assistant is added to aluminum nitride is sintered, and thus particles having no cavities and a high strength against the crushing can be obtained. However, the method of Patent Document 5 has a problem that a starting material is expensive due to use of aluminum nitride powder therefor, and also has a problem that particles are easily combined with each other due to sintering. Further, the method of Non Patent Document 1 is disadvantageous to industrial scale due to high price of its starting materials and complexity of its steps.